Intermetallic compounds are compounds of a defined structure comprising a metal and either a non-metal (metalloid) or further metal, They have many applications. For example silicon carbide is used in metal matrix composites as a strengthening additive and for furnace electrodes. Molybdenum silicide is also used as a furnace element and as a strengthening agent. Titanium diboride is used as a possible cathode material for the Hall-Heroult cell for the extraction of alumina.
Carbides are amongst the most refractory materials known. Many carbides have softening points above 3000° C. and the more refractory carbides possess some of the highest melting points ever measured. Of the simple carbides, the most refractory are HfC and TaC, which melt at 3887° C. and 3877° C. The complex carbides 4TaC.ZrC and 4TaC.HfC melt at 3932° C. and 3942° C., respectively. Silicon carbide is quite resistant to oxidation at temperatures up to about 1500° C. and has useful oxidation resistance for many purposes at temperatures up to 1600° C. It is used extensively for example as an abrasive, as a refractory and as a resistor element for electric furnaces.
Most carbides have fair thermal and electrical conductivity, and many of them are quite hard, boron carbide being the hardest. High hardness accounts for the usefulness of many of the carbides, such as silicon carbide, titanium carbide, boron carbide and tungsten carbide as materials for cutting, grinding and polishing and for parts subject to severe abrasion or wear.
Most carbides are prepared by the reaction of the oxide with carbon at elevated temperatures. Other methods of preparation include vapour deposition from the gaseous phase.
The carbides of Group II elements are usually prepared commercially by reacting the oxide with graphite in an electric-arc furnace at around 2000° C. Boron carbide and silicon carbide are made by a similar route, as are transition or hard metal carbides. High purity carbides are difficult to prepare commercially.
TiB2 and ZrB2 have potential for replacing carbon as an electrode material in aggressive electrochemical applications such as aluminium refining. Their good electrical conductivity, good wettability and excellent chemical resistance means greatly increased lifetimes. TiB2 is harder than tungsten carbide and has an excellent stiffness to weight ratio so it has important applications for cutting tools, crucibles and other corrosion resistance applications.
Boride powders can be prepared by the carbothermic or aluminothermic reduction of metal oxide-boron oxide mixtures, by electrolysis of fused salt mixtures containing metal oxides and boron oxide and by heating mixtures of metal and boron powders to high temperatures in an inert atmosphere. Fusion electrolysis is especially suited to the large-scale production of boride powders of relatively high purity from naturally occurring raw materials, and does not require the initial preparation of metal and boron powders. However, the current efficiency is very low of the order of 5%.
Of conventional methods, direct synthesis of refractory borides permits the greatest control of composition and purity of the resulting boride. However, the temperature required is very high (1700° C.).
Conventionally, silicides can be prepared by six general methods, i.e. synthesis from the elements (metal and silicon); reaction of metal oxide with silicon; reaction of metal oxide with silicon and carbon; and reaction of silica and metal oxide with carbon, aluminium or magnesium. The silicides are chemically inert, have high thermal and electrical conductivities, are hard and have high strengths at elevated temperatures coupled with high melting points.
Aluminides are made by the direct reaction of the elements.
Generally, these interesting materials are made at very high temperatures where it is difficult to ensure high purity. The electrochemical methods that have been tried generally work at very low current efficiencies.